This application claims the priority of Japanese Patent Application No. 10-347900/1998 filed on Nov. 20, 1998, which is incorporated herein by reference.
The present invention relates to a pasted hydrogen-absorbing alloy electrode for an alkaline storage battery comprising an active material layer formed on a current collector from a mixture including a hydrogen-absorbing alloy powder, a conductive agent and a binder.
An alkaline storage battery using a hydrogen-absorbing alloy electrode as a negative electrode has been recently regarded promising because of its higher energy density as compared with that of a conventional alkaline storage battery using a cadmium electrode or a zinc electrode as a negative electrode.
A pasted hydrogen-absorbing alloy electrode, which is a kind of the hydrogen-absorbing alloy electrode, is fabricated by applying a paste, obtained by mixing a hydrogen-absorbing alloy powder and a binder solution, on a current collector and drying the paste. In the pasted hydrogen-absorbing alloy electrode, electric contact among hydrogen-absorbing particles becomes defective with ease, resulting in lowering the conducting property. When the conducting property is lowered, the proportion of the hydrogen-absorbing alloy particles not involved in absorption and desorption of hydrogen (namely, charge and discharge) is increased, which can cause, in a battery using this electrode, decrease of the discharge capacity, degradation of the charge-discharge cycle performance and increase of the internal pressure during charge.
Accordingly, a carbon powder is generally included as a conductive agent in a pasted hydrogen-absorbing alloy electrode for improving the conducting property, and a technique to improve dispersibility of a carbon powder in a paste has been proposed (Japanese Laid-Open Patent Publication No. 5-307952/1993).
The oxygen absorbing power, however, cannot be sufficiently increased merely by improving the conducting property by adding a carbon powder. Therefore, the degradation of the charge-discharge cycle performance and the increase of the internal pressure derived from degradation through oxidation of the hydrogen-absorbing alloy cannot be sufficiently suppressed by this method. The improvement in the conducting property alone is not adequate for sufficiently increasing the oxygen absorbing power but oxygen should be easily adsorbed onto the surface of the hydrogen-absorbing alloy.
Furthermore, it has been recently reported that a composing element of a hydrogen-absorbing alloy can be prevented from eluting into an alkaline electrolyte by adding an oxide or a hydroxide of lanthanoid in the hydrogen-absorbing alloy or the alkaline electrolyte, resulting in improving the charge-discharge cycle performance and the storage characteristic (Japanese Laid-Open Patent Publication No. 8-222210/1996).
The oxygen absorbing power, however, cannot be sufficiently increased merely by adding an oxide or a hydroxide of lanthanoid. Therefore, the degradation of the charge-discharge cycle performance and the increase of the internal pressure derived from degradation through oxidation of the hydrogen-absorbing alloy cannot be sufficiently suppressed by this method. This is because an oxide or a hydroxide of lanthanoid does not have a function to improve the conducting property. On the contrary, the conducting property of the hydrogen-absorbing alloy particles can be degraded by singly adding an oxide or a hydroxide of lanthanoid with no conducting property, resulting in lowering the discharge capacity, in particular, in high rate discharge.
The present invention was devised in view of the aforementioned conventional problems, and an object of the invention is providing a pasted hydrogen-absorbing alloy electrode for attaining an alkaline storage battery with small increase of the internal pressure during charge, large discharge capacity in high rate discharge, and good charge-discharge cycle performance.